1. Field of the Invention
The present invention relates to a fiberoptic broadband wavelength converter used as a key device for constructing an optical communication network, particularly in WDM (wavelength division multiplexing), and a wavelength converting optical fiber and a pump source used in such an apparatus.
2. Related Background Art
It is possible to generate, from three lights having frequencies f1, f2, f3, light having frequency f4 different from these frequencies, by using FWM (four wave mixing) based on third-order nonlinear polarization in an optical fiber. In this case, the frequency f4 is determined by three frequencies f1, f2, f3 and a relationship (f4+f1=f2+f3) is established. Here, particularly, in case of f1=f2, it is referred to as xe2x80x9cDFWM (degenerated four wave mixing)xe2x80x9d, where generated light having frequency f4 is called as idler light.
The FWM and DFWM have been applied to wavelength conversion and dispersion compensation by phase conjugate light. For example, when pump having a wavelength xcexp (=c/fp) is combined by an optical fiber through which optical signal having a wavelength xcexs (=c/fs) is propagated, by a coupler, at output end of the optical fiber, the idler light is generated by DFWM, as well as the optical signal and the pump. xe2x80x9ccxe2x80x9d in xcexp (=c/fp) is a speed of light in vacuum. Since the idler light is the same as the optical signal except that it has a wavelength different from that of the optical signal and has property of phase conjugation, when the pump and the optical signal are removed from the output light from the optical fiber by using a filter to pick up only the idler light, it is possible to realize a wavelength converter of the optical signal.
Nowadays, in WDM communication, it has been attempted that a bandwidth used to the optical communication has been become broader than that of the conventional EDFA (Erbium-Doped Fiber amplifier). The bandwidth of the conventional EDFA is typically inside the region of 1530 nm-1560 nm; so-called C-band. As one example, the bandwidth has been expanded to 1570 nm-1610 nm (referred as L-band) by using specially designed EDFA, Raman amplifier and so on. Concerning the broadband WDM optical communication networks based on such amplifiers, interconnection of the two independent WDM systems that are composed of the signals inside the different wavelength region, will be required. In this situation, all-optical signal processing is required and a broadband wavelength converter is expected to enhance the flexibility of the networks. Until now, a fiberoptic broadband (36 nm half width of the half maximum) wavelength converter using fiber DFWM was reported.
It is known that the bandwidth of the fiberoptic wavelength converter using the DFWM has infinite conversion bandwidth in principle under the condition that the pump wavelength coincides with the zero-dispersion wavelength of the fiber. However, in truth the conversion bandwidth is actually limited into the finite wavelength region because of the following five obstacles.
The first obstacle is chromatic dispersion variance of the optical fiber along the longitudinal direction. It is known that inhomogeneous distribution of the zero-dispersion wavelength seriously deteriorates the conversion efficiency. In other words, efficient idler generation is not expected under the large variance of the zero-dispersion wavelength.
The second obstacle is PMD (polarization-mode dispersion) of the fiber. Because of the fiber PMD difference of the SOP (state of polarization) of the pump and signal becomes larger as the lightwaves propagates. It is well-known that it is preferable to coincide the SOP of the signal with the pump in order to generate the idler light efficiently by the FWM in the optical fiber. Further, it is also known that generation efficiency of the idler light becomes zero when the SOP of the optical signal and pump are orthogonal. However, because of the following reasons, it is difficult to coincide the SOP of the signal and pump perfectly along the entire fiber length. Even if the SOP of the optical signal and the pump are coincided carefully at the input end of the optical fiber, unless PMF (polarization maintaining fiber) is used as the optical fiber and linearly polarization along an optical axis of polarization is launched into such a fiber, phase of the incident lights are changed during the propagation. In general, since there is PMD, i.e., birefringence in the optical fiber, the polarization state is not preserved. Further, since the birefringence is small and is distributed inhomogeneously along the longitudinal direction, there is no optical axis in the practical sense. Even when generalized inherent SOP such as principal state of polarization is chosen, since the magnitude of the birefringence itself is small and is thermally unstable, a stable wavelength conversion is impossible. In general, phase change of the lightwave during propagation induces the change of the SOP. When we put difference of the wavelength between the optical signal and the pump as xcex94xcex, phase difference xcex94xcfx86 (a quantity representing difference of SOP between pump and signal) is represented as:                               Δ          ⁢                      xe2x80x83                    ⁢          φ                =                                                            -                                                      2                    ⁢                                          xe2x80x83                                        ⁢                    π                                                        λ                    p                    2                                                              ·              Δ                        ⁢                          xe2x80x83                        ⁢                          n              ·              Δ                        ⁢                          xe2x80x83                        ⁢                          λ              ·              L                                ∝                      Δλ            ·            L                                              (        3        )            
where, xcex94n is birefringence induced refractive index difference, L is the fiber length, and xcexp is a wavelength of the pump.
As can be seen from the above equation (3), the phase difference xcex94xcfx86 is proportional to both the wavelength difference xcex94xcex and the length L of the optical fiber. Accordingly, the larger the wavelength differences xcex94xcex increases, the larger the influence of change of polarization increases and it becomes more difficult to avoid the deterioration of the conversion efficiency due to PMD during the propagation. In order to solve this problem, it has been attempted that the phase difference between the pump and the signal be decreased by reducing contribution of L in the above equation (3) by using a polarization maintaining high nonlinearity optical fiber or extremely shortening the length L of the high nonlinearity optical fiber without polarization maintaining characteristics.
The third obstacle is the fact that the pump wavelength and the zero dispersion wavelength cannot be equalized exactly. Although the conversion bandwidth becomes infinite only when the wavelength of the pump is completely coincided with the zero dispersion wavelength of the optical fiber, even if they are slightly deviated from each other, the infinite converting bandwidth cannot be realized. However, for the practical sense, it is almost impossible to completely equalize the wavelength of the pump to the zero dispersion wavelength of the optical fiber.
The fourth obstacle is effect (high order effect of dispersion) of the fourth-order group velocity dispersion of the fiber. In general, in order to generate DFWM efficiently, the following phase matching conditions must be satisfied regarding both frequency of light and the propagation constant xcex2:
2xcfx89p=xcfx89s+xcfx89cxe2x80x83xe2x80x83(4)
2xcex2(xcfx89p)=xcex2(xcfx89s)+xcex2(xcfx89c)xe2x80x83xe2x80x83(5)
Where, xcfx89 is angular frequency and has a relationship between the angular frequency and the frequency f is xcfx89=2xcfx80f.
In general, when the DFWM in the optical fiber is considered, phase matching of the frequency and phase matching of the propagation constant must be satisfied simultaneously. In this case, since the phase matching of the frequency can easily be realized, we should concentrate on realizing the phase matching of the propagation constant. When the broadband wavelength conversion based on the DFWM is considered, phase mismatch xcex94xcex2 of the propagation constant xcex2 is represented as follows:                                           Δ            ⁢                          xe2x80x83                        ⁢            β                    ≡                                    β              ⁡                              (                                  ω                  c                                )                                      +                          β              ⁡                              (                                  ω                  s                                )                                      -                          2              ⁢                              β                ⁡                                  (                                      ω                    p                                    )                                                                    =                  2          ⁢                                    ∑                              m                =                1                            ∞                        ⁢                          xe2x80x83                        ⁢                                          1                                                      (                                          2                      ⁢                      m                                        )                                    !                                            ⁢                                                (                                                                                    ⅆ                                                  2                          ⁢                          m                                                                    ⁢                      β                                                              ⅆ                                              ω                                                  2                          ⁢                          m                                                                                                      )                                                  ω                  =                                      ω                    p                                                              ⁢              Δ              ⁢                              xe2x80x83                            ⁢                              ω                                  2                  ⁢                  m                                                                                        (        6        )            
Here, the frequency interval xcex94xcfx89 is as follows:
xcex94xcfx89xe2x89xa1xcfx89cxe2x88x92xcfx89p=xcfx89pxe2x88x92xcfx89sxe2x80x83xe2x80x83(7)
In this way, the phase mismatch xcex94xcex2 of the propagation constant xcex2 comprises of even-order term, and, in general, second order term (so called second order group velocity dispersion) is dominant. This term represents the chromatic dispersion coefficient at the wavelength of the pump. Thus, the DFWM can be generated efficiently by equalizing the wavelength of the pump to the zero dispersion wavelength of the fiber. Furthermore, when the SOP of the pump and the signal are coincided with each other, the bandwidth of the wavelength conversion becomes infinite theoretically. However, as described in connection with the third obstacle, in actual, it is impossible to coincide the wavelength of the pump with the zero dispersion wavelength of the optical fiber, and, even if the wavelength of the pump is slightly deviated from the zero dispersion wavelength, the bandwidth will be limited. In such a case, it is also conceived that the case of m=2 in the above equation (6), i.e., effect of fourth-order group velocity dispersion contributes to deterioration of the bandwidth.
The fifth obstacle for limiting the bandwidth of the wavelength conversion is decoherence between pump and signal. In general, in order to generate the efficient DFWM in the optical fiber, the length of the optical fiber L must be reduced smaller than the coherent length defined by the following equation (8):                               L          coh                ≡                              2            ⁢            π                                "LeftBracketingBar"            Δβ            "RightBracketingBar"                                              (        8        )            
Based on the above (8) and the equation (6) in which assuming the second-order term is dominant, the following relation can be derived, and the bandwidth of the wavelength conversion is limited by this relation:                               "LeftBracketingBar"          Δω          "RightBracketingBar"                ≤                                                            2                ⁢                π                            L                        ⁢                          "LeftBracketingBar"                                                (                                                            (                                                                                                    ⅆ                            2                                                    ⁢                          β                                                                          ⅆ                                                      ω                            2                                                                                                                "RightBracketingBar"                                    )                                                  ω                  ⁢                                      xe2x80x83                                    ⁢                  p                                                  -                  1                                                                                        (        9        )            
The bandwidth of the wavelength conversion is limited by the above-mentioned five obstacles. FIG. 1 represents a comparison of conversion efficiency spectrum actually measured by using an HNL-DSF (high nonlinearity dispersion shift optical fiber) and a result of numerical calculation performed by using parameters of such an optical fiber. The measured result is shown by the filled circle (xe2x97xaf) (a) in FIG. 1 and the numerical calculation result (theoretical value) is shown by the solid line (b) in FIG. 1. The vertical axis in FIG. 1 represents conversion efficiency and the horizontal axis represents a difference between the wavelength xcexc of the idler and the wavelength xcexp of the pump. In the measurements, the wavelength of the pump was equalized to the zero dispersion wavelength of the optical fiber. The maximum conversion efficiency was xe2x88x9214.8 dB and 3 dB bandwidth (half width) was 22.7 nm. The numerical calculation was made by numerically integrating a basic equation of DFWM obtained by considering only the linearly polarization and by regarding nonlinear polarization as scalar quantity. In this way, in the scalar approximation, influence of SOP mismatch between the optical signal and the pump is not included in the theory. This means that two polarization states always coincide with each other. In this case, in the solution of the scalar equation, when the pump wavelength is coincided with the zero dispersion wavelength, the conversion efficiency spectrum becomes flat and has the infinite bandwidth, and, thus, the solid line (b) in FIG. 1 becomes parallel to the horizontal axis. However, as shown in FIG. 1, the conversion efficiency has the finite bandwidth, and, as the xcexcxe2x88x92xcexp increases, the conversion efficiency is deteriorated. It is considered that this deterioration comes from one of the above-mentioned five obstacles or combination thereof.
The Inventors conceived optical parametric amplification to reduce the above-mentioned five obstacles to broaden the conversion wavelength bandwidth and flatten the conversion efficiency spectrum. A calculation result of the conversion efficiency obtained from the optical parametric amplification is shown in FIG. 2. The vertical axis in FIG. 2 indicates the conversion efficiency and the horizontal axis indicates a wavelength difference between the wavelength xcexc of the idler and the wavelength xcexp of the pump. FIG. 2 shows a typical example. From the figure, it is appeared that wavelength dependence of the conversion efficiency spectrum has the following properties. {circle around (1)} As wavelength interval of the pump and the optical signal are increased, the conversion efficiency of the corresponding idler light increases. {circle around (2)} Once the conversion efficiency becomes maximum when the wavelength interval of the pump and the signal reaches a certain value, even if the wavelength intervals are further increased, the conversion efficiency decreases. {circle around (3)} As the bandwidth is increased until the conversion efficiency becomes maximum, conversion efficiency increases monotonically. The property {circle around (3)} is opposite to the property of the curve (a). The measured result (measured result of the wavelength conversion using an HNL-DSF) is shown in FIG. 1. The Inventors conceived that the conversion efficiency spectrum may be flattened in a wider range by compensating for the deterioration of the wavelength conversion bandwidth caused by the above-mentioned five obstacles by using the above property {circle around (3)} of the optical parametric amplification.
To verify the above discussion the Inventors made experiment. The result is shown in FIG. 3. The vertical axis in FIG. 3 indicates the conversion efficiency and the horizontal axis indicates a difference between the wavelength xcexc of the idler and the wavelength xcexp of the pump. The result shown in FIG. 3 is obtained from an experiment of the wavelength conversion by using another HNL-DSF different from the optical fiber used in the measurement shown in FIG. 1 and by changing the wavelength of the signal. In FIG. 3, open circle (◯) (a) represents the conversion efficiency spectrum measured by coinciding the wavelength of the pump with the zero dispersion wavelength of the optical fiber, and the filled circle (xe2x97xaf) (b) represents the conversion efficiency spectrum measured while the wavelength of the pump is in anomalous dispersion region to flatten the conversion efficiency spectrum. From FIG. 3, it is verified that the deterioration of the wavelength conversion bandwidth caused by the above-mentioned five obstacles was compensated for by the optical parametric amplification. In the measurement, it was verified that, if the wavelength of the pump is shifted toward the anomalous dispersion region more than the case shown in FIG. 3, the effect of the optical parametric amplification increases and the wavelength flatness is lost.
The Inventors made further investigation and developed a fiberoptic wavelength converter (present invention) in which the property of the conversion efficiency spectrum of the optical parametric amplification and the above-mentioned property of bandwidth limitation can be cancelled by setting the pump wavelength in an optimum anomalous dispersion region of the optical fiber.
The optical parametric amplifier has broader conversion bandwidth than that of a conventional wavelength converter in which solely DFWM is generated by coinciding the wavelength of the pump with the zero dispersion wavelength.
FIG. 4 shows a measurement result obtained by using an HNL-DSF having a length of 100 m. The vertical axis in FIG. 4 indicates the conversion efficiency and the horizontal axis indicates a wavelength difference between the wavelength xcexc of the idler and the wavelength xcexp of the pump. The zero dispersion wavelength of the HNL-DSF in the measurement was 1564.2 nm. Further, average power of the pump was 24.1 dBm (257 mW). A phase modulation and an intensity modulation were both applied to the pump to avoid influence of stimulated Brillouin scattering. In FIG. 4, the result shown by ◯ is a result obtained when the wavelength of the pump is coincided with the zero dispersion wavelength of the HNL-DSF. In this case, because of the above-mentioned reasons, as the bandwidth is broadened, i.e., as the value (xcexcxe2x88x92xcexp) is increased, the conversion efficiency is deteriorated monotonically. In FIG. 4, the result shown by X points is a result obtained when the pump wavelength is set in the anomalous dispersion region to maximize the conversion efficiency at xcexcxe2x88x92xcexp=30 nm. In this case the pump wavelength was 1565.2 nm. It is apparent from the result that at region of the large value of the quantity xcexcxe2x88x92xcexp, the larger conversion efficiency is realized in the case of X in comparison of the case of ◯. However, tilt of the conversion efficiency spectrum of the two cases is totally almost equivalent. For realizing the sufficiently flat conversion efficiency spectrum, the Inventors optimized the pump wavelength to maximize the conversion efficiency at xcexcxe2x88x92xcexp=40 nm. The condition is realized when the pump wavelength is 1564.9 nm. A result of the measurement is shown by filled circle of the FIG. 4. Apparently, in comparison with the other results shown by the ◯ and X, conversion spectrum with good flatness in the broader bandwidth was realized. From the result, it is verified the optimized parametric amplification can compensate for the tilt of the conversion efficiency spectrum. Further, the dotted line a and the solid line shown in FIG. 4 are obtained by fitting the each of the above result by third-order polynomial. Evaluating from these fitting curves, in the case of xcexp=1564.9 nm, only 0.5 dB tilt inside the bandwidth of 30 nm is realized.
Next, the principle of the present invention will be explained precisely. In order to generate the optical parametric amplification, it is required that the pump is in the anomalous dispersion region of the fiber and the pump power must be larger than the threshold of the MI (modulational instability), and, in this case, the highly efficient wavelength conversion can be realized. Particularly when the intensity-dependent phase matching condition is satisfied between the wavelength of the optical signal and the wavelength of the pump, the conversion efficiency becomes maximum. The phase matching condition under the conventional DFWM is represented as follows:                     Δβ        =                                            -                                                2                  ⁢                                      πλ                    p                    2                                                  c                                      ⁢                          D              ⁡                              (                                  λ                  p                                )                                      ⁢            Δ            ⁢                          xe2x80x83                        ⁢                          f              2                                =          0                                    (        10        )            
In the case of including the effect of the optical parametric amplification, the eq. (10) is replaced as:                                                         Δ              ⁢                              xe2x80x83                            ⁢              k                        ≡                          Δβ              +                              2                ⁢                γ                ⁢                                  xe2x80x83                                ⁢                                  P                  p                                                              =                                                                      -                                                            2                      ⁢                                              πλ                        p                        2                                                              c                                                  ⁢                                  D                  ⁡                                      (                                          λ                      p                                        )                                                  ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                                  f                  2                                            +                              2                ⁢                                  xe2x80x83                                ⁢                γ                ⁢                                  xe2x80x83                                ⁢                                  P                  p                                                      =            0                          ,                            (        11        )            
where, c is speed of light in vacuum, D is chromatic dispersion coefficient of the fiber, xcex94f is a frequency difference between the signal and the pump, and xcex3 represents nonlinear coefficient of the fiber. Conversion efficiency Gc, which is defined as the ratio of the idler power and input signal power, is represented by the following equation (12):                                           G            c                    =                                                                      (                                      γ                    ⁢                                          xe2x80x83                                        ⁢                                          P                      p                                        ⁢                    L                                    )                                2                            ⁡                              [                                                      sinh                    ⁡                                          (                      gL                      )                                                        gL                                ]                                      2                          ,                            (        12        )            
where, g is parametric gain which is defined by the following equation (13):
xe2x80x83g=xc2xd{square root over (xe2x88x92xcex94xcex2(xcex94xcex2+4xcex3Pp))}=xc2xd{square root over (xe2x88x92(xcex94xcex2+2xcex3Pp)2+(2xcex3Pp)2)}xe2x80x83xe2x80x83(13)
From the above equations (12) and (13), when xe2x80x9cgxe2x80x9d is real, the parametric amplification is induced and the conversion efficiency increases exponentially with respect to L. Thus, in order to induce the parametric amplification, the following relationship (14) must be satisfied:
xcex94xcex2 less than 0 and xcex94xcex2+4xcex3Pp greater than 0xe2x80x83xe2x80x83(14)
The relationship (14) means that, in order to generate the parametric amplification, the wavelength of the pump should be in the anomalous dispersion region and the pump power must satisfy the following condition (15):                               P          p                 greater than                   -                                    Δ              ⁢                              xe2x80x83                            ⁢              β                                      4              ⁢                              xe2x80x83                            ⁢              γ                                                          (        15        )            
The right hand side of the relationship (15) is called as xe2x80x9cthreshold of MI (modulational instability)xe2x80x9d. From the equation (13), it can be seen that the conversion efficiency Gc becomes maximum when the phase matching condition shown by the equation (11) is satisfied.
From the above discussion, in order to generate the optical parametric amplification, it is required that the pump wavelength must be set to the anomalous dispersion region of the fiber and the pump power satisfies the equation (15). In this case, the highly efficient wavelength conversion can be realized. Particularly when the effective phase matching condition of the equation (11) is satisfied between the signal and the pump, the conversion efficiency becomes maximum. Based on the theory, the fact that the parametric amplification can compensate for the deterioration of the conversion efficiency in broadband region will be explained hereinbelow.
FIG. 5 shows a result of calculation of the conversion efficiency while the wavelength of pump having larger power than the threshold of MI is being gradually shifted from the zero dispersion wavelength to a longer wavelength region (anomalous dispersion region). The vertical axis in FIG. 5 indicates the conversion efficiency and the horizontal axis indicates the wavelength difference between the wavelength xcexc of the wavelength conversion light (idler) and the wavelength xcexp of the pump. In the calculation, since the zero dispersion wavelength of the optical fiber is 1559.3 nm and dispersion slope is selected to 0.07 ps/nm2/km, the longer wavelength side from the zero dispersion are becomes the anomalous dispersion region. The pump power Pp is fixed to 27 dB. From FIG. 5, the wavelength dependence of the conversion efficiency spectrum is characterized as follows. When the pump wavelength is set to be longer than the zero-dispersion wavelength of the optical fiber, the gradient of the conversion efficiency spectrum in the vicinity of xcexcxe2x88x92xcexp=0 becomes larger as the pump wavelength is longer. In addition, for a given pump power the conversion efficiency spectrum has a maximal value when xcexcxe2x88x92xcexp satisfies the condition of eq. (11). The conversion efficiency decreases monotonically over the maximal value. As shown in FIG. 5, in the vicinity of the xcexcxe2x88x92xcexp=0, the sign of the gradient of the conversion efficiency spectrum is opposite to the gradient of the tilt due to the five obstacles appeared in the preceding discussion. Consequently the tilt can be compensated for by an optimized parametric amplification induced from a suitable pump power and wavelength.
Next, a role of the pump power in the optical parametric amplification is discussed. As shown in the above equations (12) and (13), the gain of the conversion efficiency shown in FIG. 5 also depends upon power of the pump. In order to verify the fact, calculations were made with changing the pump power. The parameters of the optical fiber used in the calculation are same as used the calculation shown in FIG. 5 and the wavelength of the pump is fixed to 1565.0 nm. A calculation result is shown in FIG. 6. The vertical axis in FIG. 6 indicates the conversion efficiency and the horizontal axis indicates a difference between the wavelength xcexc of the wavelength conversion light (idler) and the wavelength xcexp of the pump.
From the results shown in FIG. 6, it can be seen that, when the pump power is small, for example when the pump power is 17 dBm and 20 dBm, the conversion efficiency simply becomes large with conserving the similar spectrum shape. However, if the pump power is increased sufficiently, the parametric amplification is generated, the spectrum shape changes as shown in FIG. 5. Accordingly, for a given fiber, to realize a sufficiently flat conversion efficiency spectrum the optimum pump power can be determined by changing the pump power in this way. In this case, the wavelength of the pump must be inside the anomalous dispersion region of the optical fiber. In principle, in the degenerated four wave mixing, since the signal frequency and the idler frequency are placed symmetrically around the pump, the conversion efficiency spectrum becomes a symmetrical shape around the pump. According to the above-mentioned theory, the present invention is embodied as follows.
The present invention relates to a fiberoptic broadband wavelength converter. According to an aspect of the present invention, the apparatus is characterized in that a pump wavelength is set in an anomalous dispersion region of an optical fiber for wavelength conversion and pump power is set to be larger than a threshold of MI so that wavelength conversion with highly flattened conversion efficiency spectrum in a wide bandwidth is realized by degenerated four wave mixing (DFWM) with optical parametric amplification.
According to another aspect of the present invention, in the fiberoptic broadband wavelength converter, a pump light source can oscillate pump having a wavelength and intensity which can flatten the conversion efficiency within the wide bandwidth inside the anomalous dispersion region of the optical fiber used for wavelength conversion.
According to a further aspect of the present invention, in the fiberoptic broadband wavelength converter, signal of a center wavelength is xcexs, it is wavelength-converted by using a pump with wavelength xcexp through DFWM. It is shown that the idler (converted wave) wavelength is represented as in the following equation (1):                               λ          c                =                                            λ              s                        ⁢                          λ              p                                                          2              ⁢                              xe2x80x83                            ⁢                              λ                s                                      -                          λ              p                                                          (        1        )            
and the wavelength of the pump is set to the wavelength represented by the following equation (2) to realize wavelength conversion from a signal wavelength xcexs to the wavelength xcexc of the optical signal after conversion:                               λ          p                =                              2            ⁢                          λ              s                        ⁢                          λ              c                                                          λ              s                        +                          λ              c                                                          (        2        )            
and the wavelength of the pump that is set in the anomalous dispersion region of the fiber to flatten the conversion efficiency spectrum within the wide bandwidth for wavelength conversion.
According to a still further aspect of the present invention, concerning the fiberoptic broadband wavelength converters, under the condition that the pump power and wavelength are both kept constant, bandwidth of the conversion efficiency is evaluated through measuring the conversion efficiency by deviating the signal wavelength from the pump wavelength, and repeating the same evaluation by changing the pump wavelength from the zero dispersion wavelength of the fiber to a wavelength inside the anomalous dispersion region, a pump wavelength at which a sufficiently flattened conversion efficiency spectrum is realized, is found, and the determined wavelength is used as the pump wavelength for broadband wavelength conversion.
According to a yet further aspect of the present invention, in the fiberoptic wavelength converter, under a condition that the pump power is kept constant, measurement of the conversion efficiency is made to determine the optimized pump wavelength in anomalous dispersion region of the fiber by changing the pump and signal wavelength with preserving the wavelength interval of the two lightwaves constant, and, a pump wavelength that realizes the maximum conversion efficiency is used as the wavelength of the pump for broadband wavelength conversion.
According to a further aspect of the present invention, in the fiberoptic broadband wavelength converter, a length of the optical fiber for wavelength conversion as medium for the DFWM is smaller than 200 m.
According to a further aspect of the present invention, in the fiberoptic broadband wavelength converter, when the wavelength of the pump is set in the anomalous dispersion region of the optical fiber for wavelength conversion and the pump power is set to be greater than the threshold, wavelength conversion which can be flattened within a wide bandwidth is realized by the degenerated four wave mixing (DFWM).